============================== Originally posted 2014-4-8 ================================== North Pole Environmental Observatory 2013 Aerial CTD Survey NSF Grants OPP-9910305, OPP-0352754, and ARC-0856330 CTD Station Position Latitude _ Longitude Cast Date / Time Cast 1 8930_80W 89deg 29.946min North 77deg 08.533min West- 2013-4-12/1331 UTC Cast 2 85_90E 84deg 57.886min North 89deg 21.925min East 2013-4-14/1132 UTC Cast 3 86_90E 85deg 59.568min North 89deg 41.740min East 2013-4-14/1532 UTC Cast 4 87_90E 87deg 00.502min North 89deg 50.410min East 2013-4-15/1152 UTC Cast 5 88_90E 88deg 02.087min North 89deg 18.390min East 2013-4-15/1543 UTC Cast 6 89_90E 88deg 58.458min North 89deg 37.531min East 2013-4-16/1021 UTC Cast 7 89_180W 88deg 57.437min North 179deg 16.094min West- 2013-4-16/1505 UTC Cast 8 88_180W 88deg 01.735min North 179deg 54.314min West- 2013-4-17/1102 UTC Cast 9 87_180W 87deg 00.427min North 179deg 38.893min East 2013-4-17/1456 UTC Cast 10 85_170W 84deg 55.356min North 169deg 50.288min West- 2013-4-18/1158 UTC Cast 11 86_175W 86deg 4.155 min North 173deg 55.305min West- 2013-4-18/1523 UTC Cast 12 90N 89deg 57.634min North 175deg 12.985min West- 2013-4-19/0910 UTC Cast 13 8830_90W 88deg 31.328min North 88deg 13.396min West- 2013-4-19/1309 UTC Each cast is an ASCII file of twelve numerical columns with a short header- _ Depth (m) _ Pressure (dbar) _ Temperature in situ (deg C) _ Potential Temperature (deg C) _ Conductivity (S/m) _ Salinity (psu) _ Density (sigma-theta) _ Dissolved Oxygen (rawV) _ Dissolved Oxygen (ml/l) _ Dissolved Oxygen (mg/l) _ Dissolved Oxygen (%sat) _ Dissolved Oxygen (Mmol/Kg) Part of the observational program of the North Pole Environmental Observatory, these CTD-chemistry stations were obtained using a Twin Otter skiplane operating out of the Russian Ice Station Barneo to record ocean sections from the North Pole along 180 degrees and 90 degrees East longitudes. The measurements were made with a Seabird SBE-19plus Seacat (s/n 5076) following a landing at these positions on the Arctic sea ice. Mounted on and plumbed together with the CTD was SBE-43 Dissolved Oxygen Sensor s/n 229. Suspended on the line above the CTD and plumbed to its pump outflow was an ISUS V2 Nitrate Sensor. Using a winch installed in the aft door of the airplane also allowed Niskin Bottles to be mounted and tripped at chosen depths on the line, and water samples from these were drawn inside the heated fuselage. Concentrations of various chemical tracers were obtained, and these and the Nitrate data are archived in a different submission. Ideally the downcasts provide better quality data than the upcasts, since they are freer of instrument wake effects and show the best resolution and detail of small features. In spring Arctic conditions with very cold air temperatures and surface water at the freezing point, care must be taken to avoid seawater freezing in the plumbing the instant it enters the water and not dissipating before reaching a substantial depth, which can result in contamination of the top of the downcast. Such precautions as pitching a heated tent over the hole and Twin Otter aft door and waiting a long period with the instrument soaking in the Mixed Layer before beginning the cast have usually been adequate. During the 2012 survey, however, these steps had turned out to be insufficient. In 2013, we approached the problem of keeping the instrument warm with its plumbing clear of obstruction with renewed determination, but once again, in five of the 13 stations, a careful eye can detect flow-dependent contamination in the upper Down-casts. At length, we decided to represent Casts 2, 3, 4, 9, and 12 by the Up-casts, and retain the higher resolution Down-casts for the other eight. Besides header labeling, the reported data levels appear in these files in the order recorded, to reinforce the user's impression of which direction the profiler was moving. For the future, our currrent thinking seems to be focusing on keeping the start of a downcast far enough below the bottom of the ice to avoid any stray ice chips or freezing crystals that might be found there brom being ingested into the plumbing. Data processing followed a modified SEASOFT recipe with certain constants determined by empirical trial. Temperature and conductivity were low-pass filtered with a time constant of 0.50 seconds, the dissolved oxygen voltage was filtered with a time constant of 0.25 seconds, and pressure was filtered with a time constant of 1.0 seconds. Alignment is the step that compensates for separate sensors with differing time constants and located at different points in the CTD plumbing by shifting individual data channels in time. Temperature was advanced relative to pressure by 0.55 seconds, a value determined by varying the temperature advance to select the value that did the best job of minimizing salinity spiking. Then a cell thermal mass correction was applied to conductivity, choosing parameters Alpha = 0.025 and Tau = 9.0 from the theoretical equations offered in Morison, et al (1994). The dissolved oxygen voltage was advanced by 4.0 seconds relative to pressure, selected from the cases where oxygen minima and maxima from available downcasts could be matched with the upcast on profile plots. Sea-Bird expects a range of oxygen advance between 2 and 7 seconds. Finally, the derived variables salinity, density, and dissolved oxygen concentration were calculated. Oxygen is offered in four different units for the user's convenience. SBE-43 Dissolved Oxygen Concentration is determined from the Sea-Bird developed Murphy-Larsen algorithm. Even as short a survey as 13 casts recorded over two weeks can show a drift in the oxygen calibration, and we expected to make use of a post calibration of the SBE-43 to compensate. Unfortunately, on return to the the manufacturer, SBE-43 s/n 229 had a lowered resolution due to corrosion of the anode assembly too low to allow post-calibration, that Sea-Bird considers a manufacturing defect. Consequently, reported dissolved oxygen concentration could only be determined using the pre-calibration, which can be presumed to drift to reduced accuracy with time. Worse, the degraded anode degrades one's confidence in the oxygen accuracy as the cast count rises, and even though the O2 concentration profiles reported here do seem qualitatively reasonable, the user is encouraged to also review the NPEO 2013 Chemistry submission, including oxygens drawn from water samples. Measurements by the Switchyard Project (http://psc.apl.washington.edu/switchyard) taken during May 2013 provide CTD and ocean chemistry coverage of the Lincoln Sea region, and are archived in submissions by Lamont-Doherty Earth Observatory and the University of Washington. Separately archived are other components of the North Pole Environmental Observatory including drifting buoys and bottom-moored pressure recorders. Deployment history, profile plots and other analysis using these data may be viewed at the NPEO website (http://psc.apl.washington.edu/northpole). Reference: Morison, J., R. Andersen, N. Larson, E. D'Asaro, and T. Boyd, 1994: The Correction for Thermal-Lag Effects in Sea-Bird CTD Data. J. Atmos. Oceanic Technol., 11, 1151-1164. Sea-Bird Electronics, Inc., 2013: "SEASOFT V2: SBE Data Processing UserŐs Manual". For further information, please contact Dr. James Morison morison@apl.washington.edu (206) 543-1394 Dr. Michael Steele mas@apl.washington.edu (206) 543-6586 Dr. Matt Alkire malkire@apl.washington.edu (206) 897-1623 Roger Andersen roger@apl.washington.edu (206) 543-1258 at Polar Science Center, Applied Physics Lab, University of Washington 1013 NE 40th, Seattle, WA 98105-6698 USA FAX (206) 616-3142